Abstract
Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. Polarons represent a crucial intermediate step populated immediately after dissociation of the excitons formed in the primary photoabsorption process. Here we present a novel approach to studying photoexcited polarons in an important photoactive oxide, ZnO, using infrared (IR) reflection-absorption spectroscopy (IRRAS) with a time resolution of 100 ms. For well-defined (10-10) oriented ZnO single-crystal substrates, we observe intense IR absorption bands at around 200 meV exhibiting a pronounced temperature dependence. On the basis of first-principles-based electronic structure calculations, we assign these features to hole polarons of intermediate coupling strength.
Highlights
Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies
Polarons are crucial for optoelectronics, photovoltaics and photochemistry[1], little is known about polaron lifetimes and binding energies (BEs)[2,3,4]
We present a novel approach to studying the photoexcited polarons in zinc oxide (ZnO) using infrared (IR) reflection–absorption spectroscopy with a time resolution of 100 ms employing the so-called rapid scan method, where interferograms are acquired in rapid succession with the mirror moving at the highest possible speeds
Summary
Despite their pronounced importance for oxide-based photochemistry, optoelectronics and photovoltaics, only fairly little is known about the polaron lifetimes and binding energies. For ZnO, a wide-bandgap metal oxide with desirable physical and chemical properties, only indirect experimental[2] and limited theoretical[4,5] information on the existence of polarons is available: theoretical description of large (weak coupling) electron polarons agrees with experimental observation[2,5]; and small (strong coupling) polarons are not stable[6] This contrasts with excitons, which have been studied in detail and whose lifetimes In well-defined (10-10) oriented ZnO single crystals, we observe pronounced absorption bands at around 200 meV in IR spectra under ultra-high vacuum conditions—a sharp feature and an absorption edge (AE)-like feature These previously unobserved strong bands exhibit a temperature-dependent lifetime, and through first-principles-based electronic structure theory calculations[4,6,13,14,15], we attribute them to a hole polaron of intermediate coupling strength. The groundbreaking observation and attribution of intermediate polarons will trigger a new era of polaron research
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